These studies are directed toward determining the molecular lesion affecting beta-globin production in patients with homozygous beta-thalassemia. Bone marrow cells from four patients with homozygous beta plus-thalassemia contained twice as many precursor beta globin mRNA molecules than did control cells suggesting that transcription of the beta globin gene is normal but that processing of the precursor may be defective. In contrast, the precursor to delta globin mRNA was present in a much lower concentration than beta precursor in both control and beta-thalassemic cells suggesting that the disparity in the amounts of Hb A and Hb A2 in human red cells occurs because of different rates of transcription of the delta and beta genes. Synthesis of beta globin mRNA was found to be normal when beta plus-thalassemic bone marrow cells were incubated in vitro with (3H) nucleotides but there was a delay in transport of radioactive globin mRNA sequences from nucleus to cytoplasm again suggesting a processing defect. Finally, RNA from two patients was subjected to electrophoresis in an agarose gel and abnormal precursor molecules were defined by a hybridization reaction. These results suggest that beta plus-thalassemia may occur because of mutations which affect RNA processing. Efforts are now directed towards purifying globin genes from patients with processing defects so that the mutations may be defined at the DNA sequence level.